In the broader sense, the present invention relates to a modification in the material properties of a semiconductor substrate made, for example, of silicon or silicon carbide, beginning from a surface of the semiconductor substrate. Modifications of this type may involve, for example, the setting of a certain conductivity or conductivity type (p or n doping).
Although it is applicable, in principle, to numerous other micromechanical or microelectronic semiconductor structures, the present invention and its underlying object are explained on the basis of micromechanical pressure sensors.
FIG. 3 shows a schematic cross-sectional view of a known semiconductor structure. In FIG. 3, reference numeral 1 designates a semiconductor substrate made of silicon, for example of the p-type, and reference numeral 5 designates a doping region on surface OF of semiconductor substrate 1, for example of the n-type, which has a depth t′ of 10 μm.
Doping regions 5 of this type are usually achieved by diffusing foreign atoms into semiconductor substrate 1 from surface OF.
In the case of a silicon semiconductor substrate 1, a source of dopants is deposited for this purpose onto the wafer surface (e.g., phosphorus glass for n doping or boron glass for p doping) and subsequently thermally driven in at a high temperature, i.e., the dopants are excited for the purpose of diffusion into silicon substrate 1 from surface OF. Alternatively, the dopants are also implantable into the wafer surface in a layer having an original thickness of typically 1 μm to 2 μm, this layer being subsequently thermally diffused deeper into silicon semiconductor substrate 1.
Diffusion processes of this type are generally limited to a relatively thin layer thickness from surface OF of semiconductor substrate 1, since foreign substances such as dopant atoms diffuse only slowly into silicon, even at very high temperatures, and therefore in practice are able to achieve depths of only typically 20 μm to 25 μm in silicon, at least within economically justifiable diffusion times. Foreign atoms, such as antimony (Sb) or germanium (Ge), etc., exist which diffuse only extraordinarily slowly, due to their large atom diameter, so that not even the specified limit of typically 20 μm to 25 μm in silicon is achievable using these foreign atoms, but instead the diffusion depths within justifiable times remain substantially below this level.
In the case of silicon carbide, a complicating factor is that diffusion itself takes place extremely slowly even at very high temperatures of 1,400° C. The silicon carbide lattice is a substantial diffusion barrier, which greatly blocks a penetration of foreign atoms and limits diffusion processes to a penetration depth of just a few micrometers.
In micromechanical applications, in particular, however, thicker layers of, for example, silicon or silicon carbide having modified layer properties, for example a modified conductivity type, are frequently required, so that the aforementioned limits of thermal diffusion processes in the bulk material are problematic. Examples may include thick monocrystalline, n-type silicon layers having a thickness of, for example, 100 μm to 200 μm on a p-type semiconductor substrate, such as those advantageously used for high-pressure sensors in silicon in connection with an electrochemical etch stop from p-type to n-type silicon. This also applies to the manufacture of thick silicon bending beam structures using an electrochemical etch stop from the back and, in plasma trench techniques, from the front, as well as to the manufacture of thin silicon films having a thickness of 100 μm to 200 μm and desired doping by electrochemical etching or anodizing up to a p-n junction. The same applies to silicon carbide in the case of media-resistant, high-temperature-compatible sensors as well as the manufacture of silicon carbide films using known smart cut methods via doping-selective electrochemical anodizing or etching methods.
Methods for manufacturing porous regions in silicon semiconductor substrates are discussed in German patent documents DE 100 32 579 A1 and DE 10 2004 036 032 A1.